1. Field of the Invention:
This invention relates to nuclear reactors, and more particularly to neutron emitting sources utilized for monitoring reactor neutron flux and for core startup purposes.
2. Description of the Prior Art:
In order to start up a nuclear core, a neutron source or emitter is typically utilized. The sources used have included those referred to as "primary sources" and "secondary sources". Primary sources are those which are made of neutron emitting isotopes in the form in which they are initially placed in the reactor core. Secondary sources are those which are made of initially non-neutron emitting materials, which become neutron emitters during operation of the reactor core. Typical of the materials utilized for sources are those including combinations of polonium and beryllium, plutonium and beryllium, antimony and beryllium, americium-beryllium and curium, and sources including californium. These sources can be relatively expensive. Because secondary sources are irradiated in-core, they are typically less expensive than the primary sources.
In addition to performing a start-up function, the sources are utilized during core shutdown to ensure the operability of monitoring and detection apparatus, such as neutron detectors aligned with the reactor core and positioned outside of the reactor vessel. This is in accordance with governmental regulations applicable to the nuclear industry which dictate that means must be provided for monitoring or otherwise measuring and maintaining control of the fission process under all operating conditions, including shutdown.
Accordingly, neutron sources for commercial reactors have been positioned within the nuclear core, and remain within the core, during at least one entire operating cycle. The sources maintain a fixed position. Small test reactors have also utilized sources driven through a hollow shielded tunnel up into the test core, which are subsequently removed at power operation. In the larger reactors, sources are inserted in selected fuel assemblies and extend within fuel assembly guide thimbles designed to receive control elements. They are therefore inserted within fuel assemblies positioned so as not to receive a control element. They are also disposed in assemblies close to the core periphery so as to be positioned close enough to activate the detection and monitoring apparatus outside of the reactor vessel. As the sources remain within an assembly for an entire core cycle, the primary sources, excluding those of californium, burn out within several fuel cycles when exposed to the high neutron flux during power operation. Californium sources have experienced longer life. In other terms, the sources are consumed by neutron induced fission and transmutation, as well as being mechanically damaged by heat, when exposed to the intense neutron flux levels characteristic of power operation.
As replacement of primary sources is costly, the secondary sources activated under the high neutron flux are used as replacements. The procedure, however, is costly not only in terms of initial source cost, but also in terms of the time and effort expended to reshuffle any type of source during maintenance or refueling operations. There is also a risk of mechanical damage of a secondary source during handling at refueling, and resulting extended down time.
It is therefore desirable to provide neutron sources for nuclear reactors which alleviate costs and time associated with replacement. The sources should desirably have an extended useful life, require minimum reshuffling, and provide adequate neutron emissions for both start-up and monitoring functions.